  Based on a recent progress in nonequilibrium statistical mechanics of
infinitely extended quantum systems, a nonequlibrium steady state (NESS) is
constructed for a single-level quantum dot interacting with two free reservoirs
under less general but more practically useful conditions than the previous
works. As an example, a model of an Ahoronov-Bohm ring with a quantum dot is
studied in detail. Then, NESS is shown to be regarded as a MacLennan-Zubarev
ensemble. A formal relation between response and correlation at NESS is derived
as well.
