  We calculate the Nyquist noise contribution to the dephasing rate
1/\tau_{nn}(omega, T) of disordered conductors in d dimensions in the regime
where the frequency omega is larger than the temperature T. For systems with a
continuous spectrum we find at zero temperature $1/ tau_{nn} (omega, 0) propto
nu_d^{-1} (omega / D)^{d/2}, which agrees qualitatively with the inelastic
quasiparticle scattering rate. Here nu_d is d-dimensional density of states,
and D is the diffusion coefficient. Because at zero frequency and finite
temperatures 1 / tau_{nn} (0, T) propto [ T / (nu_d {D}^{d/2})]^{2 / (4-d)} for
d < 2, the frequency-dependence of 1/ \tau_{nn} (omega, 0) in reduced
dimensions cannot be obtained by simply replacing T rightarrow omega in the
corresponding finite-temperature expression for 1/ tau_{nn} (0, T). We also
discuss the dephasing rate in mesoscopic systems with length L and show that
for omega < D /L^2 the spectrum is effectively continuous as far as transport
is concerned. We propose weak localization measurements of the AC conductivity
in the GHz-range to clarify the origin of the experimentally observed
zero-temperature saturation of the dephasing rate.
