  The long-time behavior of the velocity autocorrelation function in a
classical two-dimensional electric conduction system is studied by the
molecular dynamics simulation. In equilibrium, the effect of coexistence of
many-body interactions and a random potential is investigated. A crossover from
a positive tail proportional to $t^{-1}$, to a negative tail proportional to
$-t^{-2}$ is observed as the strength of the random potential increases. In
nonequilibrium, the positive tail is enhanced whereas the negative tail appears
at earlier times as an electric field increases.
