Conductance fluctuations in a quantum dot under almost periodic ac pumping

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 γτ_φ1, where 1/τ_φ is the dephasing rate induced by ac noise and γ 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)=A₀cosωt of sufficiently large intensity the variance is almost insensitive to the static magnetic field r-1=2√τ_φγ1. For the quasiperiodic ac field of the form A(t)=A₀[cos(ω₁t)+cos(ω₂t)] with ω1,2γ and γτ_φ1 we predict effect of enchancement of conductance fluctuations at commensurate frequencies ω₂/ω₁=P/Q.