function_of_scalar_z_only = @(z)integral2(@(w,l)f(z,w,l),0,L,0,@(w)w);
g = @(z)arrayfun(function_of_scalar_z_only,z);
Depending on how f is written, you may need to manually expand the scalar z.
function_of_scalar_z_only = @(z)integral(@(w,l)f(z*ones(size(w)),w,l),0,L,0,@(w)w);
g = @(z)arrayfun(function_of_scalar_z_only,z);
As the long function name suggests, that first function is only good for a scalar input. The ARRAYFUN modification "vectorizes" it so you can do all the usual things with g that you want, e.g. things like
x = linspace(zmin,zmax);
y = g(x);
plot(x,y);
