  Quantum transport in a narrow constriction, and in the presence of a
finite-range time-modulated potential, is studied. The potential is taken the
form $V(x,t) = V_{0} \theta(x)\theta(a-x)\cos(\omega t)$, with $a$ the range of
the potential and $x$ the transmission direction. As the chemical potential
$\mu$ is increasing, the dc conductance $G$ is found to exhibit dip, or peak,
structures when $\mu$ is at $n\hbar\omega$ above the threshold energy of a
subband. These structures in $G$ are found in both the small $a$ ($a \ll
\lambda_{F}$) and the large $a$ ($a \gg \lambda_{F}$) regime. The dips, which
are associated with the formation of quasi-bound states, are narrower for
smaller $a$, and for smaller $V_{0}$. The locations of these dips are
essentially fixed, with small shifts only in the case of large $V_{0}$. Our
results can be reduced to the limiting case of a delta-profile oscillating
potential when both $a$ and $V_{0}a$ are small. The assumed form of the
time-modulated potential is expected to be realized in a gate-induced potential
configuration.
