Resonant transport effects due to an impurity in a narrow constriction

Measurements of the conductance versus width of a narrow constriction reveal an isolated peak located before the turn-on of the first transverse mode and a resonant lineshape superimposed on the conductance of the mode. These observations are attributed to resonant tunneling through a single impurity ion and, in analogy to the atomic case, resonant scattering from a neutral impurity due to the formation of a quasi-bound “composite state”. The variation of the tunneling peak with temperature and magnetic field is analyzed to determine various properties of the impurity site.