LOCALIZED PLASMONS IN POINT CONTACTS

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.