  We present evidence for spin-charge separation in the tunneling spectrum of a
system consisting of two quantum wires connected by a long narrow tunnel
junction at the edge of a GaAs/AlGaAs bilayer heterostructure. Multiple
excitation velocities are detected in the system by tracing out electron
spectral peaks in the conductance dependence on the applied voltage, governing
the energy of tunneled electrons, and the magnetic field, governing the
momentum shift along the wires. The boundaries of the wires are important and
lead to a characteristic interference pattern in measurements on short
junctions. We show that the experimentally observed modulation of the
conductance oscillation amplitude as a function of the voltage bias can also be
accounted for by spin-charge separation of the elementary excitations in the
interacting wires.
