A metal-semiconductor-metal (M-S-M) model for quantitative analysis of current-voltage (I-V) characteristics of semiconducting nanowires is described and applied to fit experimental I-V curves of Bi2S3 nanowire transistors. The I-V characteristics of semiconducting nanowires are found to depend sensitively on the contacts, in particular on the Schottky barrier height and contact area, and the M-S-M model is shown to be able to reproduce all experimentally observed I-V characteristics using only few fitting variables. A procedure for decoupling contact effects from that of the intrinsic parameters of the semiconducting nanowires, such as conductivity, carrier mobility and doping concentration is proposed, demonstrated using experimental I-V curves obtained from Bi2S3 nanowires and compared with the field-effect based method.