  Quasiparticle transport in the vortex state of an s-wave superconductor at T
-> 0 was investigated by measuring the thermal conductivity of LuNi_2B_2C down
to 70 mK in a magnetic field perpendicular to the heat current. In zero field,
there is no electronic conduction, as expected for a superconducting gap
without nodes. However, as soon as vortices enter the sample quasiparticles are
seen to conduct remarkably well, even better than they would in a typical
d-wave superconductor. This is in stark conflict with the widely held view that
quasiparticle states in s-wave superconductors just above H_{c1} should be
localized and bound to the vortex core.
