Spectroscopy of the Potential Profile in a Ballistic Quantum   Constriction
  We present a theory for the nonlinear current-voltage characteristics of a ballistic quantum constriction. Nonlinear features first develop because of above-barrier reflection from the potential profile, created by impurities in the vicinity of the constriction. The nonlinearity appears on a small voltage scale and makes it possible to determine distances between impurities as well as the magnitude of the impurity potentials. 
I. E. Aronov, M. Jonson, and A. M. Zagoskin