  Reznikov et al. (Phys. Rev. Lett. 75, 3340 (1995)) have presented definitive
observations of nonequilibrium noise in a quantum point contact. Especially
puzzling is the "anomalous" peak structure of the excess noise measured at
constant current; to date it remains unexplained. We show that their experiment
directly reveals the deep link between conservation principles in the electron
gas and its low-dimensional, mesoscopic behavior. Key to that connection are
gauge invariance and the compressibility sum rule. These are central not only
to the experiment of Reznikov et al. but to the very nature of all mesoscopic
transport.
