Towards an experimental measure of the number of metastable states in spin-glasses ?

We show that an 'ergodic' time scale can be extracted from the aging experiments on spin-glasses. This is the time for which a significant fraction of the metastable states have been visited. Experiments suggest that the number of metastable states per independent subsystem is $\simeq 10^{12}-10^{14}$. For our insulating spin-glass $({\rm CdCr}_{1.7} {\rm In}_{0.3} {\rm S}_4)$ this number doubles when the temperature is lowered by 1K, but is roughly constant for the metallic sample (Ag:Mn 2.6%).