The disproportionate increase of the resistance of silicon nanowires with reduced diameter is theoretically analyzed, taking into account both the effective conducting cross section and the hole concentration. Previously reported decrease in the conducting cross section and reduction of effective hole concentration that are observed when the nanowire radius is decreased were correlated with an increased influence of the interface state density on the wire's electrical characteristics. By measuring the resistances of doped Si nanowires as a function of nanowire radii and comparing those to an analytical model developed in this study, we extracted both the doping concentration and the interface state density. The measured doping concentration of Si nanowires epitaxially bridged between a pair of doped Si electrodes was found to be 2 × 1018 cm-3 and the corresponding interface trap density is ~ 2 × 1012 cm-2 ·eV-1, which contributes to a reduced ionization of impurities resulting in a lower effective charge hole density than the doping concentration. By measuring the nanowire radius, length, and resistance, this analytic model can be used to estimate, control, and optimize the doping concentration and surface treatment method for Si nanowires for the fabrication of nanowire based devices such as field-effect transistors.