  We study the low-energy electronic transport across periodic extended defects
in graphene. In the continuum low-energy limit, such defects act as
infinitesimally thin stripes separating two regions where Dirac Hamiltonian
governs the low-energy phenomena. The behavior of these systems is defined by
the boundary condition imposed by the defect on the massless Dirac fermions. We
demonstrate how this low-energy boundary condition can be computed from the
tight-binding model of the defect line. For simplicity we consider defect lines
oriented along the zigzag direction, which requires the consideration of only
one copy of Dirac equation. Three defect lines of this kind are studied and
shown to be mappable between them: the pentagon-only, the zz(558) and the
zz(5757) defect lines. In addition, in this same limit, we calculate the
conductance across such defect lines with size L, and find it to be
proportional to k_FL at low temperatures.
