  This work investigates the quantum transport in a narrow constriction acted
upon by a finite-range transversely polarized time-dependent electric field. A
generalized scattering-matrix method is developed that has incorporated a
time-dependent mode-matching scheme. The transverse field induces coherent
inelastic scatterings that include both intersubband and intersideband
transitions. These scatterings give rise to the dc conductance $G$ a general
suppressed feature that escalates with the chemical potential. In addition,
particular suppressed features -- the dip structures -- are found in $G$. These
features are recognized as the quasi-bound-state (QBS) features that arise from
electrons making intersubband transitions to the vicinity of a subband bottom.
For the case of larger field intensities, the QBS features that involve more
photons are more evident. These QBS features are closely associated with the
singular density of states at the subband bottoms. An experimental setup is
proposed for the observation of these features.
