  We present theoretical and experimental results exploring a half-Heusler
compound TmPdSb with unusual band order and metallic surface states. Typically,
the half-Heusler systems exhibit topological features in a semimetallic state,
and trivial ones in an insulating state. Topological properties of the most of
half-Heusler systems are related to the band inversion around the Fermi level,
similar to this observed in CdTe/HgTe systems. In the case of TmPdSb we
observed the gapped electronic band structure with ``band inversion'' in the
conductance band, while the slab-like system realized metallic surface states.
The bulk insulating nature of the compound was corroborated by means of
electrical transport measurements. The experimental data revealed several
features due to the presence of metallic surface states, such as linear
magnetoresistance and weak-antilocalization effect, characterized by enhanced
coherence length and a very large number of surface conductive channels. Our
findings reveal new features of the rare-earth half-Heusler.
