Interfacial resistance plays a critical role in the transport properties of nanomaterial-based assemblies. However, understanding of them remains limited due to the difficulty of experimental approaches. Here we report in situ measurements of the electrical resistance of a single parallel contact between carbon nanotubes. By varying the contact length systematically, we derive the electrical conductivities of carbon nanotubes and interfaces. The interface between nanotubes exhibits conductivity intermediate between those of pyrolytic and single-crystal graphite. The threshold contact length between interface- and bulk-dominant electrical transport is quantitatively estimated.