Spin-resolved conductance quantum

A perfectly transmitted spin-resolved channel contributes $e^2/h$ to two-terminal linear conductance. Two degenerate spin channels give $2e^2/h$. Flux normalization cancels group velocity against the longitudinal state density.

For perfect transmission above threshold $E_n$, integrating $-df/dE$ gives $G_n=(e^2/h)f(E_n)$. Thermal occupation rounds a zero-temperature conductance step over an energy range of order $k_{B}T$.