  Double exchange mechanism is believed to favor transport along ferromagnetic
directions, the failure of which in explaining the unusual resistivity
anisotropy in iron pnictides is investigated. Several factors intrinsic to the
microscopic mechanism of transport in iron pnictides are identified and
analyzed, including the moderate Hund's coupling, low local moment, and
presence of two anisotropic degenerate orbitals xz and yz. In particular, the
substantial second neighbor hoppings are found to be decisive in giving results
opposite to the double exchange picture. In high temperature nonmagnetic phase,
orbital ordering is shown to give the right trend of resistivity anisotropy as
observed experimentally, advocating its essential role in electron transport of
iron pnictides.
