  We study mesoscopic fluctuations of the differential capacitance of a dot
connected to a lead by a single-mode quantum channel with adjustable
conductance G. We show that the amplitude of the fluctuations reaches maximum
at a partially opened channel, G\lesssim e^2/\pi\hbar. Parametric correlations
of fluctuations at different values of the applied voltage and magnetic field
can be studied experimentally, and we find the corresponding correlation
functions for the differential capacitance.
