Using a hydrodynamic model of the electron fluid in a point contact ge
ometry we show that localized plasmons are likely to exist near the co
nstriction. We attempt to relate these plasmons to the recent experime
ntal observation of deviations of the quantum point contact conductanc
e from ideal integer quantization. As a function of temperature, this
deviation exhibits an activated behaviour, 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 simp
le theoretical model involving quasiparticle lifetime broadening due t
o coupling to the localized plasmons.