  Anderson localization has been studied extensively for more than half a
century. However, while our understanding has been greatly enhanced by
calculations based on a small epsilon expansion in d = 2 + epsilon dimensions
in the framework of non-linear sigma models, those results can not be safely
extrapolated to d = 3. Here we calculate the leading scale-dependent correction
to the frequency-dependent conductivity sigma(omega) in dimensions d <= 3. At d
= 3 we find a leading correction Re{sigma(omega)} ~ |omega|, which at low
frequency is much larger than the omega^2 correction deriving from the Drude
law. We also determine the leading correction to the renormalization group
beta-function in the metallic phase at d = 3.
