Universal Multimedia Access (UMA) aims at enabling a straightforward consumption of multimedia content in heterogeneous usage environments, which may range from mobile devices in a wireless network to high-end desktop computers with wired network connectivity. Scalable video content can be used to deal with the diverse set of restrictions and capabilities of these usage environments. However, in order to optimally tailor scalable video content along the spatial, temporal, or perceptual quality axis, a quality metric is needed that reliably models subjective quality. The major contribution of this paper is the development of a novel quality metric for scalable video bit streams that are compliant with the H.264/AVC Scalable Video Coding (SVC) standard. In the context of this paper, these scalable video bit streams are intended to be used in mobile video applications, such as adaptive video streaming to mobile devices. The proposed quality metric allows modeling the temporal, spatial, and perceptual quality characteristics of SVC-compliant bit streams by taking into account several properties, such as temporal and spatial variance, frame rate, spatial resolution, and PSNR values. An extensive number of subjective experiments have been conducted to construct and verify the reliability of our quality metric. Experimental results show that the average correlation coefficient for the video sequences tested is as high as 0.96 (compared to a value of 0.51 when only using the traditional PSNR quality metric). The proposed quality metric also shows a uniformly high performance for video sequences with different temporal and spatial characteristics.