  Within condensed-matter systems, strong electronic interactions often lead to
exotic quantum phases. A recent manifestation of this is the unexpected
observation of magnetic quantum oscillations and metallic thermal transport,
both properties of systems with Fermi surfaces of itinerant quasiparticles, in
the Kondo insulators SmB6 and YbB$_{12}$. To understand these phenomena, it is
informative to study their evolution as the energy gap of the Kondo-Insulator
state is closed by a large magnetic field. We show here that both the
quantum-oscillation frequency and the cyclotron mass display a strong field
dependence in the resulting high-field metallic state in $_{12}$. By tracking
the Fermi-surface area, we conclude that the same quasiparticle band gives rise
to the quantum oscillations in both insulating and metallic states. These data
are understood most simply using a two-fluid picture where unusual
quasiparticles, contributing little or nothing to charge transport, coexist
with conventional fermions. In the metallic state this leads to a heavy-fermion
bad metal with negligible magnetoresistance, relatively high resistivity and a
very large Kadowaki-Woods ratio, underlining the exotic nature of the fermion
ensemble inhabiting $_{12}$.
