Formalisms for current-voltage (I-V) characteristics of a single molecular junction with electrodes possessing one-, two-, and three-dimensional energy distributions are derived and compared in the framework of elastic scattering theory. Model calculations on a device constituted of 1,4 benzene-dithiolate molecule chemically bonded to two gold electrodes are carried out using hybrid density functional theory. It is shown that the dimensionality of metal contacts has a strong effect on the I-V characteristics of the molecular junction, in particular the shape of the conductance curves. Good agreement with experimental I-V curves has been found for the case of the three-dimensional electron system. (C) 2003 American Institute of Physics.