  The remarkable field dependence of the electronic thermal conductivity
observed in MgB_2 can be explained as a consequence of multi-gap
superconductivity. A key point is that for moderately clean samples, the mean
free path becomes comparable to coherence length of the smaller gap over its
entire Fermi surface. In this case, quasiparticle excitations bound in vortex
cores can easily be delocalized causing a rapid rise in the thermal
conductivity at low magnetic fields. This feature is in marked contrast to that
for anisotropic or nodal gaps, where delocalization occurs only on part of the
Fermi surface.
