Resonant tunneling in a quantum waveguide: Effect of a finite-size attractive impurity

We investigate the electron transport in a quasi-one-dimensional constriction with an attractive, finite-size impurity, in the ballistic limit theoretically. Within the envelope function approximation we formulate the scattering matrix exactly that determines the resonance structure of the electron transmission. Due to the multiple impurity levels under the many-channel conditions, multiple Breit-Wigner resonances and Fano line shapes appear in the same energy window. By varying the size of the impurity, we predict novel coherent effects such as the collapse of the Fano resonance and antiresonance, resonance-level inversion, and the appearance of discrete levels in the continuum. [S0163-1829(99)07439-1].