  The surface conductivity for conduction electrons with a fixed chirality in a
topological insulator with impurities scattering is considered. The surface
excitations are described by the Weyl Hamiltonian. For a finite chemical
potential one projects out the hole band and one obtains a single electronic
band with a fixed chirality. One obtains a model of spinless electrons which
experience a half vortex when they return to the origin. As a result the
conductivity is equivalent to a spinless problem with correlated noise which
gives rise to anti-localization. We compute conductivity as a function of
frequency and compare our results with the $Raman$ shift measurement for
$Bi_{2}Se_{3}$.
