Where is the Shot Noise of a Quantum Point Contact?

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.