You may be searching for the holy grail of pants. I hope you can find something that fits your needs as I've yet to find the perfect pants for that category.
If the durability is more important, the closest I've managed was simply military BDU's (not the cotton knock offs). 511tactical.com sells them in a variety of types and they are indeed tough but the good ones are only medium or heavy weight. They are still lighter than most other even medium durability options and if it was important enough one could remove many of the pockets and features to lighten it up significantly with only a pair of nippers.
All of my attempts at light weight pants for bushwacking have ended up back at REI with holes in them. Which thanks to their 100% promises of quality has made me more willing to try some of their brands that promise to handle such things. So far, it hasn't happened despite trying a dozen pairs or more.
If light weight is more important then I've had best luck (oddly) with more mainstream companies like Columbia instead of the specialty companies such as Montbell, Mammut, OR or North Face. The columbia convertable pants last me about two seasons worth of use before I'm patching small holes, but that's better than the 1/2 of a trip for the north face ones I've tried. Maybe I'm just too hard on my pants.
I know a few members here in Australia that rely on Taslan trousers for bushwacking which may be the best option overall but they simply aren't commercially available here in the states for me to really test and comment on them. Rodger did however put together some DIY patterns in the premium content I believe.
If it's just burrs you are worried about then putting on your rain or wind shell pants will usually shed them. The downside is heat and humidity management get much harder when hiking in a shell. The down side is of course, that if it's actual branches and throns then most anything is going to get shredded.