  We present measurements of the dynamical response of thermal noise to an ac
excitation in conductors at low temperature. From the frequency dependence of
this response function - the noise thermal impedance - in the range 1 kHz-1 GHz
we obtain direct determinations of the inelastic relaxation times relevant in
metallic wires at low temperature: the electron-phonon scattering time and the
diffusion time of electrons along the wires. Combining these results with that
of resistivity provides a measurement of heat capacity of samples made of thin
film. The simplicity and reliability of this technique makes it very promising
for future applications in other systems.
