On the Bootstrap 3 media queries documentation it says:
We use the following media queries in our Less files to create the key breakpoints in our grid
The reference to 480px has been removed. Instead it says:
/* Extra small devices (phones, less than 768px) */
/* No media query since this is the default in Bootstrap */
There isn't a breakpoint below 768px in Bootstrap 3.
If you want to use the @screen-sm-min
and other mixins then you need to be compiling with LESS, see http://getbootstrap.com/css/#grid-less
Here's a tutorial on how to use Bootstrap 3 and LESS: http://www.helloerik.com/bootstrap-3-less-workflow-tutorial