  We study the transport property for armchair-edge graphene nanoribbons
(AGNRs) with an adatom coupling to a semi-infinite quantum wire. Using the
nonequilibrium Green's function approach with tight-binding approximation, we
demonstrate that the tunneling current through the system is sensitively
dependent on both the AGNR width and adatom location. Interestingly, when the
adatom locates onto a carbon atom in the 3$j$th chain from the edge of a
metallic AGNR, the system shows a transmission gap accompanied by a threshold
voltage in $I-V$ curve like a semiconducting AGNR. This effect may be useful in
scanning tunneling microscopy experimental characterization on graphene
samples.
