  A newly discovered topological insulator (AgxPb1-xSe)5(Bi2Se3)3m, m=2, has a
band gap of 0.5 eV, the largest value ever reported in topological insulators
(TIs). We present a magnetoconductivity study of the Dirac electrons of this
compound in the quantum diffusion regime. Two dimensional weak antilocalization
was observed and identified as destructive interference caused by the Berry
phase of this topological state. We find that the phase coherence length of the
Dirac electrons is independent of doping and disorder levels. This provides
proof for the absence of backscattering arising from the protection of time
reversal invariance in TI (AgxPb1-xSe)5(Bi2Se3)3m, m=2. We further report that
the homologous compound (AgxPb1-xSe)5(Bi2Se3)3m, m=1 is a superconductor with a
transition temperature Tc = 1.7 K. The related structures of these two phases
allow lateral intergrowth of crystals to occur naturally, offering an
opportunity to observe the Majorana Fermion state at the boundary of two
intergrown crystals.
