Nasal cannulae will always have outside air entrained into the inspiratory stream, diluting the oxygen. A very high flow rate would help this, but >4-5 l/m becomes very uncomfortable and uses oxygen supply quickly. Nasal reservoir bags work to some degree, but without a gas tight fit in the nare, it is not as efficient as a well fitted mask with bag. A beard dramatically reduces the efficiency of a mask for the same reason.
4 liters O2 by nasal cannula at 18,000' will result in a sea level equivalent of approx 25% oxygen, 2 liters, about 18%. The 18,000' is arbitrary, but coincides with FAA Class A airspace and is a reasonable estimate of where a cannula is unable to reasonably provide the normal 20% (sea level equivalent) O2.
An on-demand system with cannula, such as Mountain High, is superior at higher altitudes. Flow occurs only on inspiration, reducing drying, usage, also, the oxygen delivered comes early in the I/E cycle, and most of it reaches gas exchanging portions of the lung.
P