High-definition metal-oxide-metal (MOM) heterojunction nanowires in the Au-NiO-Au system have been synthesized using a template-based method. These nanowires are 70 nm in diameter and in total length, with a 100 to 300 nm wide NiO segment sandwiched between the Au nanowires axially. Detailed electron-microscopy characterization studies of these nanowires show that the oxide segment is primarily cubic NiO and nanocrystalline, and that both the Au-NiO interfaces are well-defined. These Au-NiO-Au nanowires have been incorporated into high-quality single-nanowire devices, fabricated using a direct-write method. The current-voltage responses of individual Au-NiO-Au nanowires have been measured as a function of temperature in the range 298 to 573 K. While the - response at room temperature has been found to be nonlinear, it becomes more linear and less resistive with increasing temperature. These types of MOM nanowires are likely to offer certain advantages over all-oxide nanowires in fundamental size-effect studies, and they could be potentially useful as nanoscale building blocks for multifunctional nanoelectronics of the future.