  We calculate the lifetime of low-energy electronic excitations in a
two-dimensional quantum dot near a metallic gate. We find different behaviors
depending on the relative values of the dot size, the dot-gate distance and the
Thomas-Fermi screening length within the dot. The standard Fermi liquid
behavior is obtained when the dot-gate distance is much shorter than the dot
size or when it is so large that intrinsic effects dominate. Departures from
the Fermi liquid behavior are found in the unscreened dipole case of small dots
far away from the gate, for which a Caldeira-Leggett model is applicable. At
intermediate distances, a marginal Fermi liquid is obtained if there is
sufficient screening within the dot. In these last two non-trivial cases, the
level width decays as a power law with the dot-gate distance.
