i use oldschool standard 1inch quill road stems, SR, 3T, cinelli grammo, with my oldschool 26.4 (?) clamp diameter MTB bars, like KONA and just about any made before the 31.8 clamp diameter became popular. no problem. of course, any shifters, grips, brake-clamps are not interchangeable between road bars and mtb bars, but you didn't ask, so forget i said anything.

BTW, recently i've been buying carbon flatbars from China via ebay. they come in 26.4 and 31.8 clamp diameters are light cheap, and tough. i've crashed twice in the last nine months, hard enough to do permanent damage to both my shoulders, but the good news is my carbon handlebars have come through like a champ! really.