We measured, at low applied voltage, the ac electrical resistance and capacitance of a metallic powder sample under uniaxial compression. Whatever the applied stress, the frequency-dependent resistance curves can be displayed on a master curve by an appropriate rescaling. The same property is also observed for the capacitance. A one-dimensional model shows that the strong frequency dependence of the macroscopic resistance and capacitance is a consequence of the broad contact resistance distribution between grains. No microscopic model of conduction nor any parameter related to the unknown disorder need to be assumed. Relevance to other systems like random conductor-insulator mixtures or metal-cluster compounds is discussed.