  The anomalous 0.5 and 0.7 conductance plateaus in quantum point contacts in
zero magnetic field are analyzed within a phenomenological model. The model
utilizes the Landauer-Buttiker formalism and involves enhanced spin
correlations and thermal depopulation of spin subbands. In particular we can
account for the plateau values 0.5 and 0.7, as well as the unusual temperature
and magnetic field dependences of the 0.7 plateau. Finally, the model predicts
the possibility of coexisting 0.5 and 0.7 plateaus.
