A finite-element approach, with the aid of the perturbation technique, is used to determine the complex propagation characteristics of metal-clad optical fibers. Such structures of sub-micron dimension find an important application in near-field optical scanning microscopy, a technique which has been developing rapidly in recent years, providing high-resolution imaging in both the semiconductor industry and the biological sciences. The fundamental optical modal field profiles of these structures, suitable for optical scanning microscopy applications, for both TE and TM polarisations, are investigated