We extend and generalize the notion of ensemble averaging in the context of the thermodynamics of mesoscopic systems. Two important factors determine the statistical ensemble: (i) Constraints imposed on the system as external fields are varied adiabatically; (ii) the initial preparation of the ensemble. Different procedures correspond to different experimental setups, and may result in different magnitude, sign, and Aharonov-Bohm periodicity of the quantity at hand. Adiabatic dynamical response is related to one of these static averaging procedures.

• Received 26 October 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.1976