  Lattice vacancy effects on electrical conductance of nanographite ribbon are
investigated by means of the Landauer approach using a tight binding model. In
the low-energy regime ribbons with zigzag boundary provide a single conducting
channel whose origin is connected with the presence of edge states. It is found
that the chemical potential dependence of conductance strongly depends on the
difference ($\Delta$) of the number of removed A and B sublattice sites. The
large lattice vacancy with $\Delta\neq 0$ shows $2\Delta$ zero-conductance dips
in the single-channel region, however, the large lattice vacancy with
$\Delta=0$ has no dip structure in this region. The connection between this
conductance rule and the Longuet-Higgins conjecture is also discussed.
