In this article, we present an analytical model that describes the plowing coefficient of friction for sliding, elastic-plastic contacts between a conical tip with a spherical extremity and a flat substrate. The model includes the effects of adhesion and bridges the gap between models which are based solely on dislocation activity and those based solely on interfacial effects scaling with the contact area. The Derjaguin-Muller-Toporov approximation for adhesive contact stress is used in our description of the contacts. Our model shows excellent agreement with large-scale molecular dynamics simulations and atomic force microscopy experiments of nanoscratching on copper single crystals. One important result of our study is that the model predicts coefficients of friction that are an order of magnitude higher than typically reported for nanoscale elastic contacts. Furthermore, the coefficients of friction described by the model are very close to values typical of macroscale sliding contacts.