  We study the ac-conductivity in linear response theory in the general
framework of ergodic magnetic Schr\"odinger operators. For the Anderson model,
if the Fermi energy lies in the localization regime, we prove that the
ac-conductivity is bounded by $C \nu^2 (\log \frac 1 \nu)^{d+2}$ at small
frequencies $\nu$. This is to be compared to Mott's formula, which predicts the
leading term to be $C \nu^2 (\log \frac 1 \nu)^{d+1}$.
