  It is shown that the variance of the linear dc conductance fluctuations in an
open quantum dot under a high-frequency ac pumping depends significantly on the
spectral content of the ac field. For a sufficiently strong ac field
$\gamma\tau_{\phi}<< 1$, where $1/\tau_{\phi}$ is the dephasing rate induced by
ac noise and $\gamma$ is the electron escape rate, the dc conductance
fluctuations are much stronger for the harmonic pumping than in the case of the
noise ac field of the same intensity. The reduction factor $r$ in a static
magnetic field takes the universal value of 2 only for the white--noise
pumping. For the strictly harmonic pumping $A(t)=A_{0}\cos\omega t$ of
sufficiently large intensity the variance is almost insensitive to the static
magnetic field $r-1= 2\sqrt{\tau_{\phi}\gamma} << 1$. For the quasi-periodic ac
field of the form $A(t)=A_{0} [\cos(\omega_{1} t)+\cos(\omega_{2} t)]$ with
$\omega_{1,2} >> \gamma$ and $\gamma\tau_{\phi} << 1$ we predict the novel
effect of enchancement of conductance fluctuations at commensurate frequencies
$\omega_{2}/\omega_{1}=P/Q$.
