  Low-frequency properties of a plasma are examined within the average-atom
approximation, which presumes that scattering of a conducting electron on each
atom takes place independently of other atoms. The relaxation time tau
distinguishes a high-frequency region omega tau > 1, where the single-atom
approximation is applicable explicitly, from extreme low frequencies omega tau
< 1, where, naively, the single-atom approximation is invalid. A proposed
generalization of the formalism, which takes into account many-atom collisions,
is found to be accurate in all frequency regions, from omega =0 to omega tau
>1, reproducing the Ziman formula in the static limit, results based on the
Kubo-Greenwood formula for high frequencies, and satisfying the conductivity
sum-rule precisely. The correspondence between physical processes leading to
the conventional Ohm's law and the infrared properties of QED is discussed. The
suggested average-atom approach to frequency-dependent conductivity is
illustrated by numerical calculations for the an aluminum plasma in the
temperature range 2--10 eV.
