We consider a ladder model of DNA for describing carrier transport in a fullycoherent regime through finite segments. A single orbital is associated to eachbase, and both interstrand and intrastrand overlaps are considered within thenearest-neighbor approximation. Conduction through the sugar-phosphate backboneis neglected. We study analytically and numerically the spatial extend of thecorresponding states by means of the Landauer and Lyapunov exponents. Weconclude that intrinsic-DNA correlations, arising from the natural basepairing, does not suffice to observe extended states, in contrast to previousclaims.
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