Anatomy of the drift-diffusion relationship

For drifting particles, there exists a general mobility-diffusion relation, which holds regardless of equilibrium, as a differential-geometric property of phase space. It may be associated with the fact that a force field is devoid of entropy. Near equilibrium, it entails the usual Nernst-Townsend relation, while at high fields it expresses energy balance. It greatly facilitates the calculation of nonequilibrium diffusion tensors.