# Conductance quantum

The conductance quantum, denoted by the symbol G0 is the quantized unit of electrical conductance. It is defined as:

$G_0 = \frac{2 e^2}{h}$= 7.7480917346(25)×105 S.[1]

It appears when measuring the conductance of a quantum point contact, and, more generally, is a key component of Landauer formula which relates the electrical conductance of a quantum conductor to its quantum properties. It is twice the reciprocal of the von Klitzing constant (2/RK).

Note that the conductance quantum does not mean that the conductance of any system must be an integer multiple of G0. Instead, it describes the conductance of two quantum channels (one channel for spin-up and one channel for spin-down) if the probability for transmitting an electron that enters the channel is unity, i.e. if transport through the channel is ballistic. If the transmission probability is less than unity, then the conductance of the channel is less than G0. The total conductance of a system is equal to the sum of the conductances of all the parallel quantum channels that make up the system.[2]

## Derivation

In a 1D wire adiabaticly connecting two reservoirs of potential u1 and u2, the density of states is: $dn/d \epsilon = 2/hv$ and the voltage is $V = -(u_1 - u_2)/e$. The 1D current going across is the current density: $j = -ev(u_1-u_2) dn/d \epsilon$.

$G = I/V = j/V = 2e^2/h$

## Occurrence

Quantized conductance occurs in wires that are ballistic conductors, when $\lambda \gg L$. B. J.van Wees et al first observed the effect in a point contact in 1988.[3] Carbon nanotubes have quantized conductance independent of diameter.[4] The quantum hall effect can be used to precisely measure the conductance quantum value.

## Motivation from the uncertainty principle

A simple, intuitive motivation of the conductance quantum can be made using the Heisenberg uncertainty principle, which states that the minimum energy-time uncertainty is ΔEΔt ~ h, where h is the Planck constant. The current I in a quantum channel can be expressed as e/τ, where τ is transit time and the e is electron charge. Applying a voltage V results in an energy E = eV. If we assume that the energy uncertainty is of order E and the time uncertainty is of order τ, we can write ΔEΔt ~ (eV)(e/I) ~ h. Using the fact that the electrical conductance G = I/V, this becomes G ~ e2/h.

## References

1. ^ Barry N. Taylor, Peter J. Mohr (2010). "CODATA Value: Conductance Quantum". The NIST Reference on Constants, Units, and Uncertainty. National Institute of Standards and Technology. Retrieved 2011-06-23.
2. ^ S. Datta, Electronic Transport in Mesoscopic Systems, Cambridge University Press, 1995, ISBN 0-521-59943-1
3. ^ "Quantized Conductance of Point Contacts in a Two-Dimensional Electron Gas". PRL. 1988.
4. ^ S. Frank, P. Poncharal, Z. L. Wang, and W. A. de Heer (1998). "Carbon Nanotube Quantum Resistors". Science, 280, 1744-1746.