  The effective action for the current and density is shown to satisfy an
evolution equation, the functional generalization of Callan-Symanzik equation.
The solution describes the dependence of the one-particle irreducible vertex
functions on the strength of the quenched disorder and the annealed Coulomb
interaction. The result is non-perturbative, no small parameter is assumed. The
a.c. conductivity is obtained by the numerical solution of the evolution
equation on finite lattices in the absence of the Coulomb interaction. The
static limit is performed and the conductivity is found to be vanishing beyond
a certain threshold of the impurity strength.
