  We demonstrate a general and robust method to confine on a plane strongly
diffusing submicrometer particles in water by using size tunable magnetic
channels. These virtual conduits are realized with pairs of movable Bloch walls
(BWs) located within an epitaxially grown ferrite garnet film. We show that,
once inside the magnetic conduit, the particles experience an effective local
parabolic potential in the transverse direction, while freely diffusing along
the conduit. The stiffness of the magnetic potential is determined as a
function of field amplitude which varies the width of the magnetic channel, and
precise control of the degree of confinement is demonstrated by tuning the
applied field. The magnetic conduit is then used to realize single files of
non-passing particles and to induce periodic condensation of an ensemble of
particles into parallel stripes in a completely controllable and reversible
manner.
