Relation between conductivity and transmission matrix

The dc conductance Γ of a finite system with static disorder is related to its transmission matrix t by the simple relation Γ=(e²2πℏ) Tr(t^†t). This relation is derived from the Kubo formula and is valid for any number of scattering channels with or without time-reversal symmetry. Differences between various definitions of the conductance of a finite system are discussed.