You might try ExOfficio’s Vent’r Pant ($70). They’re a very light but durable pair of pants designed with ventilation in mind. Fast-drying nylon fabric, cargo pockets, treated to repel the sun. The ExOfficio Take Over pant ($78) is similar, but a little bit heavier, with reinforcements in key areas such as the knee. Both have a "regular" cut at the ankle, but you could carry a simple elastic band or something to snug them up.
You can find similar pants from other makers. Patagonia, for instance, has the Borderless Trek pants ($75), a comfortable travel pant made from nylon with a bit of Spandex for stretch. Cotton has its advantages when it’s hot and dry—Patagonia’s Custodian Pants ($69) are made from cotton canvas that is tough but also very comfortable.
Lastly, you’d be fine with REI’s Sahara Cargo Pants ($50). Same specs as the others, really—light nylon fabric, comfortable cut, built for hot weather. Plus plenty of pockets for important stuff.