Abstract
The approximation scheme recently proposed for the dynamical conductivity and density fluctuation spectrum of a quantum particle moving in a random potential is rederived within Mori's formalism. The equations are solved by asymptotic expansions (i) for small coupling to study the corrections to the kinetic equation approach for the mobility, (ii) for strong coupling to analyse the insulator instability caused by increasing spontaneous fluctuations, and (iii) for the regime close to the Anderson phase transition from a state with metallic conductivity to an insulating one. The equations are solved numerically and an extensive discussion of the solutions is presented.