I'd think the 625-11 would be a good contender for the lightest N frame. With it's 45 caliber holes, 1 7/8" barrel, and the titanium frame, it's pretty light.
I'm pretty sure it is the .357 magnum, 327pc or 327 pc or 327 performance center model # 170245 . The scandium frame titanium cylinder only weighs 21oz.
My 2.5" 325PD has to be a contender. It sure feels mightly light when I fire it. With the snub barrel and cylinder with big .45 caliber chambers and two piece barrel, there is less metal than all others except for the 327PD perhaps. Most of the metal in that revolver is lightweight alloy.