Single molecule tunneling conductance: The temperature and length dependences controlled by conformational fluctuations

We present a model for a tunnelling conductance of a chain molecule controlled by fluctuations of relative conformations between nearest neighbour units of the chain molecule. The model leads to a simple formula which in one regime reproduces recently reported Arrhenius dependences for alkanedithiols [W. Haiss, H. van Zalinge, D. Bethel, J. Ulstrup, D.J. Schiffrin, R.J. Nichols, Faraday Discuss. 131 (19) (2005)] with an activation energy proportional to the length of the molecule, whereas in other regimes it shows activationless behaviour corresponding to the tunnelling across more rigid molecules. The general formula covers the transition between the two limits, predicting some new dependences that could be interesting to verify experimentally.