Coherent quantum transport in narrow constrictions in the presence of a finite-range longitudinally polarized time-dependent field

We have studied the quantum transport in a narrow constriction acted upon b y a finite-range longitudinally polarized time-dependent electric held. The electric field induces coherent inelastic scatterings that involve both in trasubband and intersideband transitions. Subsequently, the dc conductance G is found to exhibit suppressed features. These features are recognized as the quasi-bound-state (QBS) features that are associated with electrons ma king transitions to the vicinity of a subband bottom, of which the density of states is singular. Having valleylike instead of diplike structures, the se QBS features are different from the G characteristics for constrictions acted upon by a finite-range time-modulated potential. In addition, the sub band bottoms in the time-dependent electric-held region are shifted upward by an energy proportional to the square of the electric field and inversely proportional to the square of the frequency. This effective potential barr ier is originated from the square of the vector potential, and it leads to the interesting field-sensitive QBS features. An experimental setup is prop osed for the observation of these features. [S0163-1829(99)01827-5].