  We present new results of the ``0.7'' 2(e^2)/h structure or quasi plateau in
some of the most strongly confined point contacts so far reported. This strong
confinement is obtained by a combination of shallow etching and metal gate
deposition on modulation doped GaAs/GaAlAs heterostructures. The resulting
subband separations are up to 20 meV, and as a consequence the quantized
conductance can be followed at temperatures up to 30 K, an order of magnitude
higher than in conventional split gate devices. We observe pronounced quasi
plateaus at several of the lowest conductance steps all the way from their
formation around 1 K to 30 K, where the entire conductance quantization is
smeared out thermally. We study the deviation of the conductance from ideal
integer quantization as a function of temperature, and we find an activated
behavior, exp(-T_a/T), with a density dependent activation temperature T_a of
the order of 2 K. We analyze our results in terms of a simple theoretical model
involving scattering against plasmons in the constriction.

Mesoscale and Nanoscale Physics