  We demonstrate the danger of a naive identification of kinetic coefficients
in the free-electron response expansion in powers of an external electric field
and coefficients in the expansion of the corresponding equilibrium quantity in
powers of the fluctuating intrinsic electric field. A particular example of the
direct current generation by an AC electric field in a mesoscopic system of
non-interacting electrons vs. the Fock contribution to the equilibrium
persistent current in a system of interacting electrons is considered.
