Single electron charging of impurity sites visualized by scanning gate   experiments on a quantum point contact
  A quantum point contact (QPC) patterned on a two-dimensional electron gas is investigated with a scanning gate setup operated at a temperature of 300 mK. The conductance of the point contact is recorded while the local potential is modified by scanning the tip. Single electron charging of impurities induced by the local potential is observed as a stepwise conductance change of the constriction. By selectively changing the state of some of these impurities, it is possible to observe changes in transmission resonances of the QPC. The location of such impurities is determined, and their density is estimated to be below 50 per \mu m^2, corresponding to less than 1 % of the doping concentration. 
A. Pioda, S. Kicin, D. Brunner, T. Ihn, M. Sigrist, K. Ensslin, M.   Reinwald and W. Wegscheider