How to erase contour beyond a line or multiple lines?

Hi. I have contour of a function. I want to plot this contour over a triangle domain. I have the equations of the triangle boundary. So I think I must plot contour at first and then erase contour beyond the boundary lines. But I don't know how to do it. This is my code:

clc
clear all
T=100;
a=0.3;
syms y z real
tauxy=5.1966*T*z*(2*sqrt(3)*y+a)/a^5;
tauxz=5.1966*T*(-sqrt(3)*y^2+sqrt(3)*z^2+a*y)/a^5;
tau=sqrt(tauxy^2+tauxz^2);
z1=(sqrt(3)*y-a)/3;
z2=-z1;
figure(1)
ezcontour(tau,[-a/(2*sqrt(3)),a/sqrt(3),-0.16,0.16]);
hold on
ezplot(z1,[-a/(2*sqrt(3)) a/sqrt(3)]);
ezplot(z2,[-a/(2*sqrt(3)) a/sqrt(3)]);
axis([-a/(2*sqrt(3)),a/sqrt(3),-0.16,0.16]);

can anyone help me to do this job? if you can show me a better way to plot the contour restricted between these lines then I will be thankful. Also I want to know how to plot more contour lines in the triangle domain since most of the lines are out of the boundaries.