  We consider a ladder model of DNA for describing carrier transport in a fully
coherent regime through finite segments. A single orbital is associated to each
base, and both interstrand and intrastrand overlaps are considered within the
nearest-neighbor approximation. Conduction through the sugar-phosphate backbone
is neglected. We study analytically and numerically the spatial extend of the
corresponding states by means of the Landauer and Lyapunov exponents. We
conclude that intrinsic-DNA correlations, arising from the natural base
pairing, does not suffice to observe extended states, in contrast to previous
claims.
