  We investigate theoretically the electronic transport properties in narrow
graphene ribbons with an adatom-induced defect. It is found that the lowest
conductance step of a metallic graphene nanoribbon may develop a dip even down
to zero at certain values of the Fermi energy due to the defect. Accompanying
the occurrence of the conductance dip, a loop current develops around the
defect. We show how the properties of the conductance dip depend on the
parameters of the defect, such as the relative position and severity of the
defect as well as the width and edges of the graphene ribbons. In particular,
for metallic armchair-edges graphene nanoribbons, whether the conductance dip
appears or not, they can be controlled by choosing the position of the single
defect.
