I consider myself in the ultralight/lightweight end of the backpacking spectrum, but I do prefer a fixed blade when out.
This is what I'm using these days: Fallkniven WM1. The complete package of knife, Kiahdex sheath, paracord belt loops, and LMF Army firesteel weighs 168.6g/5.9oz. You could get a lower weight without the firesteel and a less robust sheath; the knife itself weighs (spec'd) 70g/2.5oz.
If I'm going somewhere where I think I may want a bigger knife, then the Fallkniven F1 goes. Same set up as pictured below but minus the pouch and pouch anchor.
That set-up weighs 308.8g/10.9oz.
I like the Fallknivens for the corrosion-resistance while having decent wear-resistance and toughness, and the WM1 is a very light knife while still offering great blade length and profile as well as a usable handle. The thermorun handles are also nice to use when it's cold out. The carbidized Ti knives such as the one Daniel Fairly posted earlier are also good options (I had an excellent one made by Ban Tang), but I prefer the fine edge steel can provide. Mora's are also a great affordable option, and definitely light.
I also carry a custom SAK and a Leatherman Style CS for the excellent scissors.
A good knife is not something I'm willing to give up even on backpacking trips. Most folks are probably carrying a few extra pounds around the waist...drop that and you'll more than make up for taking whatever knife you wish.