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We investigate the properties of the modes that are supported by 3-D subwavelength plasmonic slot waveguides. We first show that the fundamental mode that is supported by a symmetric plasmonic slot waveguide, which is composed of a subwavelength slot in a thin metallic film embedded in an infinite homogeneous dielectric, is always a bound mode for any combination of operating wavelength and waveguide parameters. Its modal fields are highly confined over a wavelength range extending from zero frequency to the ultraviolet. We then show that for an asymmetric plasmonic slot waveguide, in which the surrounding dielectric media above and below the metal film are different, there may exist a cutoff slot width and/or a cutoff metal film thickness above which the mode becomes leaky, and there always exists a cutoff wavelength above which the mode becomes leaky. We investigate in detail the effect of variations of the parameters of the symmetric and asymmetric plasmonic slot waveguides. We also consider related alternative 3-D plasmonic waveguide geometries, such as a plasmonic slot waveguide, in which the two metal film regions that form the slot have a finite width, and a plasmonic strip waveguide, which is formed between a metallic strip and a metallic substrate. We show that for a specific modal size, the fundamental mode of the standard plasmonic slot waveguide has a larger propagation length compared with the corresponding modes of these plasmonic waveguides.