Landauer formula

$G(\mu) = G_0 \sum_n T_n (\mu) \ ,$
where $G$ is the electrical conductance, $G_0 = e^2/(\pi\hbar) \approx 7.75\times 10^{-5} \Omega^{-1}$ is the conductance quantum, $T_n$ are the transmission eigenvalues of the channels, and the sum runs over all transport channels in the conductor. This formula is very simple and physically sensible: The conductance of a nanoscale conductor is given by the sum of all the transmission possibilities an electron has when propagating with an energy equal to the chemical potential, $E=\mu$.