Plasmon spectra in a quasi-1D quantum wire under a transverse magnetic
field are calculated within the random phase approximation (RPA) in t
he long-wavelength limit. We find interesting collective behaviours co
rresponding to the edge state electrons in both intrasubband and inter
subband (between occupied subbands) excitations. The various plasmon m
odes show anomalous magnetic field dependences. The intrasubband and s
ome segments of the intersubband excitation frequencies decrease with
increasing field. These results are in good agreement with experiment.