  We have investigated experimentally and theoretically the far-infrared (FIR)
absorption of gated, deep-mesa-etched GaAs/Al$_x$Ga$_{1-x}$As quantum wires. To
overcome Kohn's theorem we have in particular prepared double-layered wires and
studied the acoustic magnetoplasmon branch. We find oscillations in the
magnetic-field dispersion of the acoustic plasmon which are traced back to the
self-consistently screened density profile in its dependence on the magnetic
depopulation of the one-dimensional subbands.
