[XeTeX] Long dash "--" in Latex

[Atip Asvanund]

Using some fonts, "--" will appear as a long dash, however with some other
fonts "--" will appear as double regular dashes.

I assume that is because "--" is not available with all fonts?

Is there a work around that would either display "--" as a single regular
dash in the fonts without the long dash, or would find a substitute from a
differnt font? For example, how would I get this to work with Warnock Pro?

Thank you,

Atip