  Using a hydrodynamic model of the electron fluid in a point contact geometry
we show that localized plasmons are likely to exist near the constriction. We
attempt to relate these plasmons with the recent experimental observation of
deviations of the quantum point contact conductance from ideal integer
quantization. As a function of temperature this deviation exhibits an activated
behavior, exp(-T_a/T), with a density dependent activation temperature T_a of
the order of 2 K. We suggest that T_a can be identified with the energy needed
to excite localized plasmons, and we discuss the conductance deviations in
terms of a simple theoretical model involving quasiparticle lifetime broadening
due to coupling to the localized plasmons.
