  Ballistic quantum wires are exposed to longitudinal profiles of perpendicular
magnetic fields composed of a spike (magnetic barrier) and a homogeneous part.
An asymmetric magnetoconductance peak as a function of the homogeneous magnetic
field is found, comprising quantized conductance steps in the interval where
the homogeneous magnetic field and the magnetic barrier have identical
polarities, and a characteristic shoulder with several resonances in the
interval of opposite polarities. The observations are interpreted in terms of
inhomogeneous diamagnetic shifts of the quantum wire modes leading to magnetic
confinement.
