  We summarize the main results of a microscopically based kinetic theory,
applicable to open quantum point contacts (QPCs) driven up to high fields. The
governing role of gauge invariance -- and the many-body sum rules for the
electron gas -- lead to stringent constraints on both transport and
fluctuations. These constraints exert a dominant influence on the observable
properties of QPCs and similar open mesoscopic conductors. We illustrate this
in the context of certain predictions within purely phenomenological models of
mesoscopic transport.
