  The performance of low temperature detectors utilizing thermal effects is
determined by their energy relaxation properties. Usually, heat transport
experiments in mesoscopic structures are carried out in the steady-state, where
temperature gradients do not change in time. Here, we present an experimental
study of dynamic thermal relaxation in a mesoscopic system -- thin metallic
film. We find that the thermal relaxation of hot electrons in copper and silver
films is characterized by several time constants, and that the annealing of the
films changes them. In most cases, two time constants are observed, and we can
model the system by introducing an additional thermal reservoir coupled to the
film electrons. We determine the specific heat of this reservoir and its
coupling to the electrons. The experiments point at the importance of grain
structure on the thermal relaxation of electrons in metallic films.
