AbstractThe question whether thermal conductivity may be time dependent on short time scales is investigated and answered in the affirmative. For this purpose, we make use of a classical relation between the Brownian mean displacement and the thermal conductivity. Interestingly, the transition from the time dependent regime to stationary behaviour is in the vicinity of Allen's temperature relaxation time. Calculation of the electronic temperature, when the new time dependent thermal conductivity is used, shows an improvement in the results derived by means of the two‐temperature model. Furthermore, the Wiedemann–Franz law is not valid in the time regime investigated. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)