  We report an electron transport study of lithographically fabricated graphene
nanoribbons of various widths and lengths at different temperatures. At the
charge neutrality point, a length-independent transport gap forms whose size is
inversely proportional to the width. In this gap, electron is localized, and
charge transport exhibits a transition between simple thermally activated
behavior at higher temperatures and a variable range hopping at lower
temperatures. By varying the geometric capacitance through the addition of top
gates, we find that charging effects constitute a significant portion of the
activation energy.
