  The current noise density S of a conductor in equilibrium, the Johnson noise,
is determined by its temperature T: S=4kTG with G the conductance. The sample's
noise temperature Tn=S/(4kG) generalizes T for a system out of equilibrium. We
introduce the "noise thermal impedance" of a sample as the amplitude of the
oscillation of Tn when heated by an oscillating power. For a macroscopic
sample, it is the usual thermal impedance. We show for a diffusive wire how
this (complex) frequency-dependent quantity gives access to the electron-phonon
interaction time in a long wire and to the diffusion time in a shorter one, and
how its real part may also give access to the electron-electron inelastic time.
These times are not simply accessible from the frequency dependence of S
itself.
