How to plot contours only inside ellips?

a=4 %input('half of the ellips major axis')

b=3 %input('half of the ellips minor axis')

Mt=1 %input('torque')

G=80 %input('regidity modulus')

x=-a:0.01:a;

y=-b:0.01:b;

y1= b*(sqrt(1-(x/a).^2));

y2 = transpose(y);

w1=y2*x;

w= (Mt*(b^2 - a^2) * w1 )/(pi * a^3 * b^3 * G);

plot(x,y1,'w',x,-y1,'w');

hold on

contourf(x,y2,w,30)

colorbar

colormap(hot)