  We study the low-frequency behavior of the {\it ac} conductivity
$\sigma(\omega)$ of a two-dimensional fermion gas subject to a smooth random
potential (RP) or random magnetic field (RMF). We find a non-analytic
$\propto|\omega|$ correction to ${\rm Re} \sigma$, which corresponds to a
$1/t^2$ long-time tail in the velocity correlation function. This contribution
is induced by return processes neglected in Boltzmann transport theory. The
prefactor of this $|\omega|$-term is positive and proportional to $(d/l)^2$ for
RP, while it is of opposite sign and proportional to $d/l$ in the weak RMF
case, where $l$ is the mean free path and $d$ the disorder correlation length.
This non-analytic correction also exists in the strong RMF regime, when the
transport is of a percolating nature. The analytical results are supported and
complemented by numerical simulations.
