  The effect of a time-modulated gate-voltage on the differential conductance
$G$ of a saddle-point constriction is studied. The constriction is modeled by a
symmetric saddle-point potential and the time-modulated gate-voltage is
represented by a potential of the form $V_{0} \theta(a/2-|x-x_{c}|) \cos
(\omega t)$. For $\hbar\omega$ less than half of the transverse subband energy
level spacing, gate-voltage-assisted (suppressed) feature occurs when the
chemical potential $\mu$ is less (greater) than but close to the threshold
energy of a subband. As $\mu$ increases, $G$ is found to exhibit,
alternatively, the assisted and the suppressed feature. For larger
$\hbar\omega$, these two features may overlap with one another. Dip structures
are found in the suppressed regime. Mini-steps are found in the assisted regime
only when the gate-voltage covers region far enough away from the center of the
constriction.
