A bridge with several independent levels

Beyond the onset of conduction, its voltage dependence is associated with the molecular level structure, though overlap between levels and experimental noise wash out much of this structure as shown schematically in figure 17.10. 

The semiconducting character of molecular junctions, expressed by the low conduction regime at low bias followed by a conduction threshold, and the subsequent conduction spectrum beyond this threshold, are manifestations of the discrete quantum level structure of small molecular systems. However, the small system nature of molecular junctions inevitably expresses itself in the appearance of noise and relatively poor reproducibility. 

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