  We report the temperature dependence of the zero-bias conductance of a
single-electron transistor in the regime of weak coupling between the quantum
dot and the leads. The Fano line shape, convoluted with thermal broadening,
provides a good fit to the observed asymmetric Coulomb charging peaks. However,
the width of the peaks increases more rapidly than expected from the thermal
broadening of the Fermi distribution in a temperature range for which Fano
interference is unaffected. The intrinsic width of the resonance extracted from
the fits increases approximately quadratically with temperature. Above about
600 mK the asymmetry of the peaks decreases, suggesting that phase coherence
necessary for Fano interference is reduced with increasing temperature.
