The if conditions can only be evaluated as paired comparisons in order to get the result you want.
Try this:
e=0:200:55000;
for i=1:length(e)
if e(i)<=11000
T(i)=(-71.5/11000).*e(i)+15;
elseif (11000<e(i)) & (e(i)<20100)
T(i)=-56.5;
elseif (20100<e(i)) & (e(i)<32200)
T(i)=(12/12100).*e(i)-76.434;
elseif (32200<e(i)) & (e(i)<47300)
T(i)=(42/15100).*e(i)-134.063;
else
T(i)=-2.5;
end
end
This has to do with the way logical comparisons are evaluated.
.