  We have conducted temperature and frequency dependent transport measurements
in amorphous Nb_x Si_{1-x} samples in the insulating regime. We find a
temperature dependent dc conductivity consistent with variable range hopping in
a Coulomb glass. The frequency dependent response in the millimeter-wave
frequency range can be described by the expression $sigma(omega) \propto
(-\imath omega)^alpha$ with the exponent somewhat smaller than one. Our ac
results are not consistent with extant theories for the hopping transport.
