where t is time and the brackets h• • • i denote an isotropic angular average over the Fermi surface. The time interval _t → ∞, so the velocity of the electron at time 0 is uncorrelated with its velocity at the earlier time −_t. This allows us to neglect at x(−_t) the small deviations from an isotropic velocity distribution induced by the density gradient [which could not have been neglected at x(0)]. Since only the time difference matters in the velocity correlation function, one has hvx(0)vx(−t)i =hvx(t)vx(0)i. We thus obtain for the diffusion constant D = −jx/c the familiar linear response formula 95